The invention relates to a process for manufacturing a fluid filter having a first filter region made of a fluid permeable material, including a number of fold walls arranged as a substantially zig-zag shaped continuous fold having at least one folded edge portion including a plurality of folded edges and adhesive interconnecting the fold walls at the folded edge portion.
Fluid filters of the above kind are used to isolate impurities from a fluid passing through them (especially air or industrial gases but basically also liquids). Filter elements are mainly used at present in air filters which are made of micro fibre fleece, especially glass fibres.
In order to increase the effective filter area, with respect to the area of the oncoming flow, of the air filter forming a filter element or a filter region, the filtering medium is folded in a zig-zag fashion so that a number of folds is produced which adjoin one another, via folded edges, through which the medium to be purified passes, at an acute angle to the walls of the folds and transversely to the direction of the folded edges.
Because of material, especially larger particles, depositing on the side of the filter facing the flow and turbulences forming in the oncoming fluid, and because of slight irregularities in the folds etc., there are inhomogeneities in the flow of the fluid passing through the filter which subject the folds arranged in this way to a great mechanical alternating stress and increased flow resistance. Furthermore, the fluid stream, when leaving the filter, is highly turbulent, the flow resistance in successive pipes and the stress on successive components being thereby increased.
In order to keep the folds at a defined spacing from each other and to stabilise the arrangement of folds in the filter inset mechanically, the folds are provided with projections protruding from the plane of the walls thereof, so that the projections of walls adjoining one another abut and the walls support each other as disclosed, for example, in U.S. Pat. No. 3,531,920 or in a special further development described in German Patent Document DE 41 29 126 A1.
It is also known from DE 40 38 966 that separate spacers, which have the same function, may be mounted on top or inserted in the walls of the folds. One variant of the spacers, as mentioned in the above document, and also in German Patent Document DE 30 37 019 A1, may be adhesive members especially in the form of threads. Apart from acting as spacers, they also connect the walls of the folds, thereby further increasing the stability of the filter inset.
Before the filter material is folded, the adhesive is applied onto the flat sheet and the walls of the folds are then stuck together when the adhesive threads come into contact with one another during folding. This solution, therefore, is only suitable for those folds which are packed closely together,
German Patent Document DE 39 03 730 A1 suggests that an adhesive thread, which stabilises and connects the folded layers with one another and which may be attached to the edge region after folding, may be combined with a stamping provided in the fold walls. This solution makes it possible to have larger spacings between the folds. There are, however, still restrictions because of the limited stamping depth of conventional filter materials.
In German Patent Document DE 42 06 407, the edge region of the folds of an already folded sheet of filter material is covered with a fine web of adhesive threads to interconnect the folds and stabilise the filter insert. This solution is not suitable in practice for larger spacings between the folds either, since the fine adhesive web is not able to provide a sufficiently stable connection of widely spaced edges and the web threads begin to "sag", especially when applied from below onto a sheet of folds positioned on top during manufacturing and are no longer in a defined position with respect to the folded edges.
European Patent Document EP 0377 420 A1 discloses an arrangement in which the top and bottom regions of the walls of the folds, each of which have localised adhesive members and are stuck together, are provided additionally with a flat layer of a finely meshed material, while utilising the adhesive provided in the top region, said material serving to render the flow more uniform and further stabilising the filter insert. The document also mentions the possibility of applying such an additional layer of material onto both edge regions of the filter insert. Again, the neutral fixing of the walls of the folds is achieved by the adhesive elements of adjacent walls coming into contact during folding, so that this solution is only suitable for closely packed folds.